1. Field of the Invention
The present invention is directed to a shock wave generator for generating an acoustical shock wave pulse, and in particular to such a shock wave generator wherein the shock wave pulse is generated by a membrane which is rapidly repelled by a coil, and which has an acoustic lens for concentrating the shock wave.
2. Description of the Prior Art
A shock wave generator is disclosed in German OS No. 33 28 051 (corresponding to the U.S. Pat. No. 4,679,505) of the type generally referred to as a "shock wave tube" for lithotripsy. The shock wave generator includes an electrical coil and a metal membrane separated therefrom by an insulating foil. When a capacitor is suddenly discharged through the coil, electromagnetic forces are generated which cause the metal membrane to rapidly repelled from the coil resulting in the emission of a shock wave pulse. An approach path or fluid path is provided adjacent to the metal membrane through which the shock wave pulse travels along a path to a patient. An acoustic lens is disposed in the approach path which operates as a focusing means to concentrate the incoming shock wave at the focus of the lens, which is preferably coincident with the position of a calculus to be disintegrated in the patient.
In practice, different treatment requirements arise from patient to patient. Such treatment conditions can vary, for example, in dependence on the type and size of the calculus to be disintegrated, the geometry of the calculus (for example, round or oblong), the distance of the calculus from the skin surface (defined by the position of the stone or the physical bulk of the patient), and on the number of calculi. In addition to these different initial conditions due to different patients, different conditions are also present as a result of the progress of the shock wave treatment. For example, during an initial treatment relatively large fragments of the calculus are first generated in a relatively wide effective zone, and these fragments must then be comminuted into smaller particles in a subsequent treatment in a more narrowly prescribed effective zone.
In order to adapt the shock wave generator to these different treatment conditions, it has been heretofore necessary to either vary the discharge voltage of the capacitor, or to vary the distance of the focusing means from the skin of the patient. These measures are not sufficient to fully adapt the shock wave generator to the large number of different treatment conditions in a satisfactory manner.